Acute cadmium toxicity in the medicinal leech <i>Hirudo verbana</i> Carena, 1820 with a novel biomarker for aquatic risk assessment

<b>Cite: </b>Çelik, Z., Aydın, H., Ceylan, M. (2026). Acute cadmium toxicity in the medicinal leech <i>Hirudo verbana</i> Carena, 1820 with a novel biomarker for aquatic risk assessment. Ecotoxicology, 35, 3. https://doi.org/10.1007/s10646-025-02996-4<b>Abstract: </b>Cadmium (Cd) is a common aquatic contaminant that poses significant ecological risks by disrupting organismal functions and ecosystem balance. This study presents the first detailed evaluation of acute Cd toxicity in the medicinal leech <i>Hirudo verbana</i> Carena, 1820, a species of ecological, biomedical, and conservation importance listed under CITES. In a 96-hour semi-static bioassay with graded Cd exposures (0–32 mg/L), we determined an LC₅₀ of 4.02 mg/L and observed a range of concentration-dependent behavioral and pathological responses, including erratic locomotion, excessive mucus secretion, regurgitation, and penis prolapse. We introduced the Body Flexion Ratio (BFR) to quantify morphological responses to Cd-induced neurotoxic stress in leeches. Measured in dead individuals due to mobility challenges in live leeches, BFR reflects current toxic responses rather than predicting Cd toxicity. A positive correlation between BFR values and Cd concentrations suggests its potential applicability as a non-behavioral indicator in field-based ecotoxicological assessments. Concurrently, Cd exposure led to a marked decline in dissolved oxygen levels and increases in conductivity, total dissolved solids, and salinity, further intensifying physiological stress in exposed organisms. These abiotic shifts correlated with the intensity of toxic responses, implying a synergistic impact of chemical and environmental stressors. This study underscores the utility of medicinal leeches as bioindicators in freshwater ecotoxicology and highlights the necessity of integrating biotic responses, including BFR, with water chemistry and ecosystem health indicators for a more comprehensive and robust risk assessment. Given the dual pressures of habitat degradation and commercial harvesting, urgent conservation measures are warranted. Our findings contribute essential toxicity thresholds and highlight <i>H. verbana</i> as a model for studying sublethal pollutant effects in shallow freshwater ecosystems.